Is it possible to pass in a tuple into a function as long as their types match up?
When I try it, I get a missing argument in parameter error:
var myTuple = ("Text",10,"More Text")
func myFunction(a:String, b:Int, c:String) {
// etc...
}
myFunction(myTuple)
It was possible, although was deprecated in Swift 2.2:
In Swift 2.1 and earlier it was possible to use a carefully crafted tuple to fill the parameters of a function. So, if you had a function that took two parameters, you could call it with a two-element tuple as long as the tuple had the correct types and element names.
...
This syntax — affectionately called “tuple splat syntax” — is the antithesis of idiomatic Swift’s self-documenting, readable style, and so it’s deprecated in Swift 2.2.
https://swift.org/blog/swift-2-2-new-features/
I came here wanting to know how to pass a tuple as a function parameter. The answers here focus on a different case. I'm not entirely clear what the OP was after.
In any case, here is how to pass a tuple as a parameter. And, for good measure, how to do it variadically.
func acceptTuple(tuple : (Int, String)) {
print("The Int is: \(tuple.0)")
print("The String is '\(tuple.1)'")
}
acceptTuple((45, "zebras"))
// Outputs:
// The Int is: 45
// The String is 'zebras'
func acceptTuples(tuples : (Int, String) ...) {
var index = 0
// note: you can't use the (index, tuple) pattern in the for loop,
// the compiler thinks you're trying to unpack the tuple, hence
/// use of a manual index
for tuple in tuples {
print("[\(index)] - Int is: \(tuple.0)")
print("[\(index)] - String is '\(tuple.1)'")
index++
}
}
acceptTuples((45, "zebras"), (17, "armadillos"), (12, "caterpillars"))
//Outputs
//[0] - Int is: 45
//[0] - String is 'zebras'
//[1] - Int is: 17
//[1] - String is 'armadillos'
//[2] - Int is: 12
//[2] - String is 'caterpillars'
Passing tuples in can be a quick and convenient approach, saving you from having to create wrappers etc. For example, I have a use case where I am passing a set of tokens and parameters to create a game level. Tuples makes this nice and compact:
// function signature
class func makeLevel(target: String, tokens: (TokenType, String)...) -> GameLevel
// The function is in the class Level. TokenType here is an Enum.
// example use:
let level = Level("Zoo Station", tokens:
(.Label, "Zebra"),
(.Bat, "LeftShape"),
(.RayTube, "HighPowered"),
(.Bat, "RightShape"),
(.GravityWell, "4"),
(.Accelerator, "Alpha"))
Yes, it's possible under these conditions:
the tuple must be immutable
the number of values in the tuple, their type, and their order must match the parameters expected by the function
named parameters must match external names in the function signature
non-named parameters must match parameters without external name in the function signature
So, your code is ok, the only thing you have to do is turning the tuple into an immutable one (i.e. using let and not var):
let myTuple = ("Text", 10, "More Text")
func myFunction(a:String, b:Int, c:String) {
// etc...
}
myFunction(myTuple)
One more example with external names:
let myTuple = ("Text", paramB: 10, paramC: "More Text")
func myFunction(a:String, paramB b:Int, paramC c:String) {
// etc...
}
myFunction(myTuple)
In your tuple, it appears as though you must name them and then refer to them as such:
so your code should be
var myTuple = (val1: "Text", val2: 10, val3: "More Text")
func myFunction(a:String, b:Int, c:String) {
// etc...
}
myFunction(myTuple.val1, myTuple.val2, myTuple.val3)
The tuple has named values (val1, val2, val3) which you set and then reference, when you pass in myTuple, to the function myFunction(), it appears as though you are just filling 1 of the 3 available arguements - and with the wrong type to boot! This is the equivalent of storing the types in a tuple, then taking them out for a function call. However, if you want a function to actually take a tuple as a parameter, see below:
var myTuple = (val1: "Text", val2: 10, val3: "More Text")
func tupleFunc(a:(String, Int, String)) {
}
tupleFunc(myTuple)
Yes, but that's the wrong structure: you're passing three variables called a, b, and c rather than a tuple with those components.
You need parentheses around the whole thing:
var myTuple = ("Text", 10, "More Text")
func myFunction(a:(x: String, y: Int, z: String)) {
println(a)
}
myFunction(myTuple)
You can use the following feature: Swift allows you to pass a function (f1) with any number of parameters (but without inout parameters) as a parameter of type (TIn) -> TOut to another function. In this case, TIn will represent a tuple from the parameters of the function f1:
precedencegroup ApplyArgumentPrecedence {
higherThan: BitwiseShiftPrecedence
}
infix operator <- :ApplyArgumentPrecedence
func <-<TIn, TOut>(f: ((TIn) -> TOut), arg: TIn) -> TOut {
return f(arg)
}
func sum(_ a: Int, _ b: Int) -> Int {
return a + b
}
print(sum <- (40, 2))
In swift 3.0, we should not able to pass the tuple directly to the function.If we did so, it shows the error message as "This type has been removed in swift 3.0"
func sum(x: Int, y: Int) -> Int
return x+y }
let params = (x: 1, y: 1)
let x = params.0
let y = params.1
sum(x: x, y: y)
Hope it helps you!!
The best option for now seems to be to just save it to a compound variable or use the build in dot syntax
let (val1, val2) = (1, 2)
func f(first: Int, second: Int) { }
f(first: val1, second: val2)
let vals = (1, 2)
f(first: vals.0, second: vals.1)
That feature called implicit tuple splat was removed in swift 3.
You can find more detailed explanation on the removal proposal here
Some suggestions to keep using tuple as an argument is by doing so:
func f1(_ a : (Int, Int)) { ... }
let x = (1, 2)
f1(x)
func f2<T>(_ a : T) -> T { ... }
let x = (1, 2)
f2(x)
Related
I am trying to learn Swift and I have bought a book to help me learn the language.
I can't understand why the following does not work:
func sum(_a: Int, _b: Int) -> Int {
return a + b
}
func subtract(_a: Int, _b: Int) -> Int{
return a - b
}
var someFunc: (Int, Int) -> Int
someFunc = sum
print(someFunc(5, 4))
someFunc = subtract
print(someFunc (5, 4))
The error I get is Cannot find 'a' (or 'b') in scope.
If I remove the underscores it does give the right answer.
I thought the point of the underscore was that the underscore meant that nothing is assigned to it and the function you want to call that returns a result but you don't care about the returned value.
Can somebody explain in simple language why this does not work.
You have to put a space between _ a, _ b. Like this:
func sum(_ a: Int, _ b: Int) -> Int {
return a + b
}
func subtract(_ a: Int, _ b: Int) -> Int{
return a - b
}
var someFunc: (Int, Int) -> Int
someFunc = sum
print(someFunc(5, 4))
someFunc = subtract
print(someFunc (5, 4))
You can learn these functions in a Playground. You will see there errors, etc...
The underscore means that the parameter will not have a label when calling then function making it more compact.
Below is 3 different ways to use labels for parameters when creating a function
Function with anonymous (no label) parameters
func example1(_ a: Int, _ b: Int) -> Int {
a + b
}
let sum = example1(3, 5)
Function with parameters as labels
func example2(a: Int, b: Int) -> Int {
a + b
}
let sum = example2(a: 3, b: 5)
Function with different parameter names and labels
func example3(first a: Int, second b: Int) -> Int {
a + b
}
let sum = example3(first: 3, second: 5)
When declaring functions in Swift, you can provide names for parameters passed to a function. It is handy to make your functions more readable when using in code. E.g. you can declare it like so:
func move(from startPoint: Int, to endPoint: Int) -> Int {
return startPoint + endPoint
}
In your function's body you can use variables' names, which is quite understandable for you. And usage of your functions will look like:
res = move(from: 1, to: 2)
I this way it will be more readable for any person who will read or use your code. It will be more clear even for you, when you'll return to this code some time later.
You can also declare the function in the way not to show any variables' names at all. For this you can use "_" as a variable's name. And this is the case from your learning book.
Guys have already answered your question. I just wanted to give you a bit deeper understanding.
In swift, you can pass functions as parameters to functions accepting closures. This is particularly useful to avoid syntactically polluting your code when using operators. For instance, you can write a sum as follows:
let values = 0 ..< 10
let sum = values.reduce(0, +)
Unfortunately, overloaded functions can lead to ambiguous situations when Swift’s inference is unable to determine the type of the expected closure from other arguments. Consider the code below for instance. The last line does not compile because Swift cannot decide what “version” of + I am referring to.
func castAndCombine<T, U>(_ pair: (Any, Any), with fn: (T, T) -> U) -> U? {
guard let first = pair.0 as? T, let second = pair.1 as? T
else { return nil }
return fn(first, second)
}
// The following line cannot compile.
let x = castAndCombine((1, 2), with: +)
Unfortunately, there isn’t (or at least I am not aware of) any way to specify which + I mean. Nonetheless, I came up with two solutions to this problem:
Add a parameter to the function to disambiguate the situation:
func castAndCombine<T, U>(_ pair: (Any, Any), toType: T.Type, with fn: (T, T) -> U) -> U? {
// ...
}
let x = castAndCombine((1, 2), toType: Int.self, with: +)
Leave the function’s signature unchanged and use a closure with explicit type annotations:
func castAndCombine<T, U>(_ pair: (Any, Any), with fn: (T, T) -> U) -> U? {
// ...
}
let x = castAndCombine((1, 2), with: { (a: Int, b: Int) in a + b })
I personally dislike the first solution, as I feel it is not aesthetic and unnatural to use. However, I wonder if the second one adds any performance overhead, due to the creation of a closure that essentially wraps a single function, without adding any behavior.
Does anyone know if this performance overhead does actually exist and/or is significant to any extent?
There should not be any overhead if you compile with optimizations, as the compiler will most likely inline your closure.
You can verify this assumption with your first solution (as it supports both styles) by comparing the LLVM code Swift writes. LLVM is an intermediate representation used by the compiler right before creating actual machine code.
Write one file using the operator directly, i.e.:
let x = castAndCombine((1, 2), toType: Int.self, with: +)
Write a second file using the closure, i.e.:
let x = castAndCombine((1, 2), toType: Int.self, with: { (a: Int, b: Int) in a + b })
Now compile both with optimizations, asking Swift's compiler to produce the LLVM IR. Assuming your files are named main1.swift and main2.swift, you can run the following:
swift -O -emit-ir main1.swift 1>main1.ll
swift -O -emit-ir main2.swift 1>main2.ll
Both produced files should be identical.
diff main1.ll main2.ll
# No output
Note that the solutions suggested in the comments do not add any performance overhead either, as statically guaranteed casts do not cost any operation.
Instead of creating a closure to disambiguate the type, you can cast + to the desired type:
func castAndCombine<T, U>(_ pair: (Any, Any), with fn: (T, T) -> U) -> U? {
guard let first = pair.0 as? T, let second = pair.1 as? T
else { return nil }
return fn(first, second)
}
// Add two Ints by concatenating them as Strings
func +(_ lhs: Int, _ rhs: Int) -> String {
return "\(lhs)\(rhs)"
}
if let x = castAndCombine((1, 2), with: (+) as (Int, Int) -> String) {
print(x)
}
12
if let x = castAndCombine((1, 2), with: (+) as (Int, Int) -> Int) {
print(x)
}
3
The code below compiles and runs OK, and seems to indicate that the closure and String.init(describing:) functions are completely equivalent in their signature, since .map method happily takes both of them.
let someDict: [String: String] = [
"string1" : "Hello",
"string2" : "Bye",
]
//One way to call .map
var closure = { (key: String, value: String) -> String in
return "The key is \(key), the value is \(value)"
}
someDict.map(closure)
//Another way to call .map
someDict.map(String.init(describing:))
But how is it possible to place into .map a String.init(describing:) function which is a function of only 1 argument, while .map expects a function of 2 arguments? Or am i misunderstanding something here..
Btw, checking the documentation shows that it really does expect a function of 2 arguments:
transform: ((key: String, value: String)) throws -> T
Btw, checking the documentation shows that it really does expect a
function of 2 arguments:
transform: ((key: String, value: String)) throws -> T
Actually, no. Notice the extra parentheses (). It shows that it expects a function that takes one argument which is a tuple containing two elements.
Consider this example:
// function foo takes two arguments
func foo(_ a: Int, _ b: Int) -> Int {
return a + b
}
// function bar takes one tuple with two elements
func bar(_ a: (Int, Int)) -> Int {
return a.0 + a.1
}
let f1 = foo
print(type(of: f1)) // (Int, Int) -> Int
let f2 = bar
print(type(of: f2)) // ((Int, Int)) -> Int
So, the extra parentheses tell us that map is expecting one argument that is a tuple containing two elements.
The closure passed to map always operates on a single element from the sequence at a time. That element can be a tuple such as your case, and then your closure can deconstruct that tuple into multiple values.
Consider this example:
// tup is a tuple containing 3 values
let tup = (1, true, "hello")
// deconstruct the tuple through assignment
let (x, y, z) = tup
print(x) // 1
print(y) // true
print(z) // hello
So in this example:
var closure = { (key: String, value: String) -> String in
return "The key is \(key), the value is \(value)"
}
someDict.map(closure)
map's closure is given a tuple of the form (key: String, value: String) and the closure is deconstructing that into key and value just as the let did above.
In this example:
someDict.map(String.init(describing:))
which is equivalent to:
someDict.map({ String(describing: $0) })
map is taking the whole tuple and passing it to String(describing:).
Is there anyway to add a new element to a tuple?
var tuple = (v1: 1,v2: 2)
tuple.v3 = 3 // Error
"error: value of tuple type '(v1: Int, v2: Int)' has no member 'v3'"
No. Each tuple with a different number of elements or a different type of elements represents a different type in the Swift type system. Hence, once you create a tuple, you cannot append elements to it, because that would change the type of the tuple.
Some really basic examples of tuples and their types:
let tupleWithTwoInts = (1,2) //has type (Int,Int)
let tupleWithThreeInts = (1,2,3) //has type (Int,Int,Int)
let tupleWithTwoStrings = ("a","b") //has type (String,String)
let tupleWithIntAndString = (1,"a") //has type (Int,String)
let tupleWithStringAndInt = ("a",1) //has type (String,Int)
Even the order of the elements make a difference in the type of a tuple.
type(of: tupleWithIntAndString) == type(of: tupleWithStringAndInt) //false
If you just need to support some small number of operations, then you can write a function for it.
For example, extending a 2D vector into 3D:
func expand(_ v: (x: Int, y: Int), z: Int) -> (x: Int, y: Int, z: Int) {
return (x: v.x, y: v.y, z: z)
}
While there is no extensible way to convert your array to a tuple as stated above, if you arrived at this question because you are making reusable components, and you like the convenience that variadic declarations can provide, you can move your method body into one that takes an array and let your variadic declaration be an alternate wrapper for the same method.
This
func buttonModels(_ buttonModels: ButtonModel...) {
self.buttonModels = buttonModels
}
Becomes:
func buttonModels(_ buttonModels: ButtonModel...) {
self.buttonModels(buttonModels)
}
func buttonModels(_ buttonModels: [ButtonModel]) {
self.buttonModels = buttonModels
}
It seems to me that you are looking to use a dictionary, not a tuple. See the docs: Tuple
EDIT: As pointed out, tuples can indeed be mutable if you declare them as a var and can be used to hold editable data. My bad!
With the code below, I get "Type '(int, int)' does not conform to protocol 'IntegerLiteralConvertible' instead of missing argument as one would expect. What's IntegerLiteralConvertible and why do you think the compiler produces this error instead for the code below?
I have looked at other SO posts regarding this error but have not gotten any insight from them.
func add(x:Int, y:Int) {
}
add(3)
My best guess is that it tries to convert the (3) tuple into a (Int, Int) tuple.
In fact, this is accepted by the compiler and works as expected:
func add(x: Int, y: Int) -> Int {
return x + y
}
let tuple = (4, 7)
add(tuple)
In playground that outputs 11, which is the expected sum result.
Note: the code above works if the func is global, with no named parameters. If it's an instance or class/static method, then the tuple must include parameter names:
class MyClass {
class func add(# x: Int, y: Int) -> Int {
return x + y
}
}
let tuple = (x: 3, y: 7)
MyClass.add(tuple) // returns 10
As for IntegerLiteralConvertible, it's used to make a class or struct adopting it to be initializable from a literal integer. Let's say you have a struct and you want to be able to instantiate by assigning an literal integer, you achieve it this way:
struct MyDataType : IntegerLiteralConvertible {
var value: Int
static func convertFromIntegerLiteral(value: IntegerLiteralType) -> MyDataType {
return MyDataType(value: value)
}
init(value: Int) {
self.value = value
}
}
and then you can create an instance like this:
let x: MyDataType = 5
It looks like you are trying to use currying — Swift has built-in support for this, but it is not automatic, so you have to be explicit about it when declaring your function:
func add(x:Int)(y:Int) -> Int {
return x + y
}
println(add(3)) // (Function)